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After studying this chapter, you should be able to: Discuss basic concepts related to concurrency, such as race conditions, OS concerns, and mutual exclusion requirements; understand hardware approaches to supporting mutual exclusion; define and explain semaphores; define and explain monitors.
Module 19: Protection • • • • • • • Goals of Protection Domain of Protection Access Matrix Implementation of Access Matrix Revocation of Access Rights Capability-Based Systems Language-Based Protection 19.1 Silberschatz and Galvin 1999 Protection • Operating system consists of a collection of object|s, hardware or software • Each object has a unique name and can be accessed through a well-defined set of operations • Protection problem - ensure that each object is accessed correctly and only by those processes that are allowed to so 19.2 Silberschatz and Galvin 1999 Domain Structure • Access-right = Rights-set is a subset of all valid operations that can be performed on the object • Domain = set of access-rights 19.3 Silberschatz and Galvin 1999 Domain Implementation • System consists of domains: – User – Supervisor • UNIX – Domain = user-id – Domain switch accomplished via file system Each file has associated with it a domain bit (setuid bit) When file is executed and setuid = on, then user-id is set to owner of the file being executed When execution completes user-id is reset 19.4 Silberschatz and Galvin 1999 Multics Rings • • Let Di and Dj be any two domain rings If j < I Di Dj 19.5 Silberschatz and Galvin 1999 Access Matrix Figure 19.6 Silberschatz and Galvin 1999 Use of Access Matrix • If a process in Domain Di tries to “op” on object Oj, then “op” must be in the access matrix • Can be expanded to dynamic protection – Operations to add, delete access rights – Special access rights: owner of Oi copy op from Oi to Oj control – Di can modify Djs access rights transfer – switch from domain Di to Dj 19.7 Silberschatz and Galvin 1999 Use of Access Matrix (Cont.) • Access matrix design separates mechanism from policy – Mechanism Operating system provides Access-matrix + rules If ensures that the matrix is only manipulated by authorized agents and that rules are strictly enforced – Policy User dictates policy Who can access what object and in what mode 19.8 Silberschatz and Galvin 1999 Implementation of Access Matrix • Each column = Access-control list for one object Defines who can perform what operation Domain = Read, Write Domain = Read Domain = Read • Each Row = Capability List (like a key) Fore each domain, what operations allowed on what objects Object – Read Object – Read, Write, Execute Object – Read, Write, Delete, Copy 19.9 Silberschatz and Galvin 1999 Access Matrix of Figure With Domains as Objects Figure 19.10 Silberschatz and Galvin 1999 Access Matrix with Copy Rights 19.11 Silberschatz and Galvin 1999 Access Matrix With Owner Rights 19.12 Silberschatz and Galvin 1999 Modified Access Matrix of Figure 19.13 Silberschatz and Galvin 1999 Revocation of Access Rights • Access List – Delete access rights from access list – Simple – Immediate • Capability List – Scheme required to locate capability in the system before capability can be revoked – Reacquisition – Back-pointers – Indirection – Keys 19.14 Silberschatz and Galvin 1999 Capability-Based Systems • Hydra – Fixed set of access rights known to and interpreted by the system – Interpretation of user-defined rights performed solely by user's program; system provides access protection for use of these rights • Cambridge CAP System – Data capability - provides standard read, write, execute of individual storage segments associated with object – Software capability -interpretation left to the subsystem, through its protected procedures 19.15 Silberschatz and Galvin 1999 Language-Based Protection • Specification of protection in a programming language allows the high-level description of policies for the allocation and use of resources • Language implementation can provide software for protection enforcement when automatic hardware-supported checking is unavailable • Interpret protection specifications to generate calls on whatever protection system is provided by the hardware and the operating system 19.16 Silberschatz and Galvin 1999 ... only by those processes that are allowed to so 19. 2 Silberschatz and Galvin 199 9 Domain Structure • Access-right = Rights-set is a subset of all valid operations that... access-rights 19. 3 Silberschatz and Galvin 199 9 Domain Implementation • System consists of domains: – User – Supervisor • UNIX – Domain = user-id – Domain switch accomplished via file system. .. Copy 19. 9 Silberschatz and Galvin 199 9 Access Matrix of Figure With Domains as Objects Figure 19. 10 Silberschatz and Galvin 199 9 Access Matrix with Copy Rights 19. 11 Silberschatz and Galvin 199 9